U.S. patent number 6,782,700 [Application Number 10/785,319] was granted by the patent office on 2004-08-31 for transient temperature control system and method for preventing destructive collisions in free piston machines.
This patent grant is currently assigned to Sunpower, Inc.. Invention is credited to Ezekiel S. Holliday, Reuven Z-M Unger.
United States Patent |
6,782,700 |
Unger , et al. |
August 31, 2004 |
Transient temperature control system and method for preventing
destructive collisions in free piston machines
Abstract
The free piston cooler transient temperature control system of
the present invention eliminates collisions during the transient
cool-down period in a free piston cooler upon start-up. The
transient temperature control system incorporates a free piston
cooler, having a cold head and a warm end, a cold head temperature
sensor, a relational interface, and a temperature controller. The
cold head temperature sensor senses the temperature of the cold
head and generates a temperature signal. The relational interface
is in communication with the temperature signal and contains a
predetermined relationship between the cold head temperature and a
maximum piston stroke during the transient cool-down temperature
range. The relational interface generates a transient range maximum
allowable stroke signal from the temperature signal and the
predetermined relationship. The temperature controller is in
communication with the relational interface and limits the piston
stroke during the transient cool-down temperature range to prevent
collisions.
Inventors: |
Unger; Reuven Z-M (Athens,
OH), Holliday; Ezekiel S. (Belpre, OH) |
Assignee: |
Sunpower, Inc. (Athens,
OH)
|
Family
ID: |
32909043 |
Appl.
No.: |
10/785,319 |
Filed: |
February 24, 2004 |
Current U.S.
Class: |
60/517;
60/524 |
Current CPC
Class: |
F25B
9/14 (20130101); F25B 2309/001 (20130101); F25B
2309/1428 (20130101); F25B 2500/26 (20130101) |
Current International
Class: |
F25B
9/14 (20060101); F01B 029/10 () |
Field of
Search: |
;60/517,518,523,524 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nguyen; Hoang
Attorney, Agent or Firm: Foster; Frank H. Kremblas, Foster,
Phillips & Pollick
Claims
What is claimed is:
1. A transient temperature control system for preventing collisions
during a transient cool-down temperature range of a free piston
cooler from a first operating temperature of a cold head to a set
point operating temperature, the cooler having a piston
reciprocating in linear oscillation within a cylinder at a variable
stroke, the system comprising: (a) a cold head temperature sensor
for sensing the temperature of the cold head and generating a
temperature signal; (b) a relational interface in communication
with the temperature signal and containing a predetermined
relationship between the cold head temperature and a maximum piston
stroke during the transient cool-down temperature range, the
relational interface generating a transient range maximum allowable
stroke signal from the temperature signal and the predetermined
relationship; and (c) a temperature controller in communication
with the relational interface, capable of receiving the transient
range maximum allowable stroke signal and limiting the stroke to
prevent collisions within the cooler during the transient cool-down
temperature range, and capable of controlling the stroke of the
piston while the cold head operates at approximately a steady state
cold head temperature.
2. The control system in accordance with claim 1, wherein the
predetermined relationship between the cold head temperature and
the stroke comprises a plurality of stored data experimentally
determined by operating the cooler during the transient cool-down
temperature range, and recording the stroke resulting in collision
at a plurality of cold head temperatures and generating a transient
controlled stroke by applying a stroke reduction factor to the
collision stroke.
3. The control system in accordance with claim 2, wherein the
plurality of stored data are resident in a table form for reference
by the relational interface in generating the transient range
maximum allowable stroke signal.
4. The control system in accordance with claim 2, wherein the
plurality of stored data are resident as a stored algorithm for
reference by the relational interface in generating the transient
range maximum allowable stroke signal.
5. A method for preventing collisions during a transient cool-down
temperature range of a free piston cooler from a first operating
temperature of a cold head to a set point operating temperature,
the cooler having a piston reciprocating in linear oscillation
within a cylinder at a variable stroke, the method comprising: (a)
sensing the temperature of the cold head and generating a
temperature signal; (b) generating a transient range maximum
allowable stroke signal in response to the temperature signal and a
predetermined relationship between the cold head temperature and a
maximum piston stroke during the transient cool-down temperature
range; and (c) limiting the stroke of the piston during the
transient cool-down temperature range, to prevent collisions within
the cooler, in response to the transient range maximum allowable
stroke signal.
6. The method in accordance with claim 5, further including the
step of experimentally developing the predetermined relationship
between the cold head temperature and the stroke by operating the
cooler during the transient cool-down temperature range, and
recording the stroke resulting in collision at a plurality of cold
head temperatures and generating a transient controlled stroke by
applying a stroke reduction factor to collision stroke.
7. The method in accordance with claim 6, further including the
step of referencing an electronic database, having the
predetermined relationship between the cold head temperature and
the stroke, when generating the transient range maximum allowable
stroke signal.
8. The method in accordance with claim 6, further including the
step of referencing an algorithm, having the predetermined
relationship between the cold head temperature and the stroke, when
generating the transient range maximum allowable stroke signal.
9. An apparatus for preventing collisions during a transient
cool-down temperature range of a free piston cooler from a first
operating temperature of a cold head to a set point operating
temperature, the cooler having a piston reciprocating in linear
oscillation within a cylinder at a variable stroke, the apparatus
comprising: (a) a means for sensing the temperature of the cold
head and generating a temperature signal; (b) a means for
generating a transient range maximum allowable stroke signal from
the temperature signal and a predetermined relationship between the
cold head temperature and a maximum piston stroke during the
transient cool-down temperature range; and (c) a means for
controlling the stroke of the piston during the transient cool-down
temperature range, to prevent collisions within the cooler, from
the transient range maximum allowable stroke signal, and
controlling the stroke of the piston during approximately steady
state cold head temperature conditions to prevent collisions within
the cooler.
10. The apparatus in accordance with claim 9, wherein the
predetermined relationship between the cold head temperature and
the stroke comprises a plurality of stored data experimentally
determined by operating the cooler during the transient cool-down
temperature range, and recording the stroke resulting in collision
at a plurality of cold head temperatures and generating a transient
controlled stroke by applying a stroke reduction factor to the
collision stroke.
11. The apparatus in accordance with claim 10, wherein the
plurality of stored data are resident in a table form for reference
by the generating means in generating the transient range maximum
allowable stroke signal.
12. The control system in accordance with claim 10, wherein the
plurality of stored data are resident as a stored algorithm for
reference by the generating means in generating the transient range
maximum allowable stroke signal.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of free piston machine
control systems, and more particularly relates to a transient
temperature range control system for avoiding damaging collisions
within a free piston cooler when operating in the cool-down
transient temperature range between start-up and steady state
operation.
2. Description of the Related Art
The Stirling engine was invented in the early 1800's but was not
used as a refrigeration cycle until 1834 when John Hershel used a
closed cycle Stirling engine to make ice. The basic concepts of the
free piston engine and the free piston cooler were invented in the
1960's by William T. Beale and are shown in U.S. Pat. No. Re30,176.
Many advances, including those in bearing technology, low clearance
seals, and regenerative materials, have vastly improved the
reliability and efficiency of free piston machines.
The free piston cooler is essentially a pressure vessel enclosing a
piston and displacer reciprocating in one or, more typically,
separate, coaxial cylinders. The piston is driven in linear
reciprocation and alternately compresses and expands a working gas
to create a pressure wave as a function of time. The displacer is
driven in reciprocation by the pressure wave acting upon a net
pressure differential area and alternately moves or shuttles a
major portion of the working gas between a cold head, where thermal
energy may be extracted from a cold head environment, and a warm
end, where heat is rejected to a warm end environment. The piston
and displacer are phased so that the piston expands the working gas
when the major part of the working gas is in the cold head and
compresses the working gas when the major part of the working gas
in at he warm end. Thus, heat is absorbed at the cold head during
expansion of the working gas and heat is rejected at the warm end
during compression of the working gas. There are, of course,
various alternative drive arrangements for moving the piston and
displacer in the desired reciprocation at the desired phase
relation.
The piston is free because no mechanical linkage confines the
piston to a fixed path of reciprocation. The free piston is
typically driven in reciprocation by a linear electric motor.
Typically, in order to maximize the efficient use of available
drive power, free piston machines are driven at their frequency of
mechanical resonance. Because the piston is unconfined in a free
piston machine, the amplitude of reciprocation, also referred to as
the stroke, is a function of piston drive force and varies under
the influence of changing operating conditions. Consequently, the
piston, as well as any other reciprocating structures, can collide
at either end of the piston stroke with physical structures at the
end of the cylinder.
In particular, in such freely reciprocating machines the amplitude
and frequency of reciprocation are a function of inertia, damping,
and spring and driving forces. Therefore, these machines share the
common feature that, when they are overdriven or underdamped, the
reciprocating parts can acquire an amplitude of reciprocation that
exceeds the internal geometric limits of the space available for
the motion of the reciprocating parts. If the amplitude of
reciprocation is allowed to increase beyond these limits, the
reciprocating parts will collide repeatedly with stationary
structures, or even with other reciprocating parts. Such collisions
are obviously undesirable because they may damage the machine.
Of particular concern, with respect to the present invention, is
the initial, transient cool-down period when the Stirling cooler
first begins operation. The period of time from the start of
operation of a free piston cooler until the cold head reaches a
desired set point temperature is the transient cool-down period.
The initial temperature of the cold head during the transient
cool-down period is often room temperature, approximately
300.degree. K, and the desired set point temperature at which the
cooler will eventually operate is often extremely cold, perhaps as
low as approximately 77.degree. K, or colder. As the working gas
transitions from the warm initial cold head temperature to the cold
set point cold head temperature, the properties of the working gas
change greatly, thereby affecting the machine operating dynamics
during the transient cool-down period. Specifically, as the working
fluid cools it becomes more dense and viscous and, therefore, the
damping of the reciprocating displacer and piston increases
accordingly as the operating temperature decreases. For example,
the density and viscosity of the working fluid may increase by a
factor of 4.
This density change creates a problem at start up during the
transient cool-down period because the control system is typically
a feedback control system designed to control piston stroke under
normal operating conditions, i.e. at the lower operating
temperature, where damping from the working gas is greater.
Consequently, at start up the cooling demand is maximum, the
control system tends to drive the piston at the drive force and
power which would be appropriate for the lower operating
temperature but the damping is less during start up than at the
operating temperature for which the control system was designed.
Therefore, unless other provision is made, the piston will be
overdriven at the warmer, temperatures where damping is less and
collisions can result.
One prior art solution is to under power the piston drive during
the transient cool-down period to assure that collisions do not
occur. The piston drive amplitude is gradually or incrementally
increased, sometimes manually, until, over a sufficient time
period, the operating temperature is reached.
However, the problem with prior art solutions is that, not only is
it desirable to avoid such collisions during the transient
cool-down period, but also it is desirable to reach the desired
operating temperature as quickly and efficiently as possible.
Therefore, for all the intermediate temperatures throughout the
entire transient cool-down period it is desirable to operate at the
maximum stroke that will not result in collisions in order to pump
heat from the cold head at the maximum rate of heat transfer and
thereby bring the cooler to its steady state operating temperature
as soon as possible.
Numerous prior art control systems have been developed to prevent
collisions from occurring within free piston machines after they
reach their normal operating temperature. The driving force or
power applied to the piston to force it in its maximum
reciprocating linear oscillation is initially much less than that
required when the colder set point operating temperature is
reached, in part because of the increase of the working gas density
as the machine cools down. Accordingly, conventional feedback
control systems designed to prevent collisions when operating at
steady state temperatures, i.e. not in the transient cool-down
period, would allow too much driving force to be applied to the
piston during the transient cool-down period, thereby promoting
collisions during that period. Such control systems have failed to
address the unique conditions presented upon start-up of the
machine. Such prior art control systems are generally only
effective once the machine has reached steady state temperature
operation.
To accomplish a maximization of stroke over the temperature range
as the temperature of the cold head decreases, the drive must be
progressively increased as the temperature is reduced and
therefore, the limit must be progressively increased. A difficulty
of doing this in an automated control system arises because there
is no known algorithm or relationship applicable to each machine
for relating the cold head temperature to maximum piston drive
power.
For purposes of describing the invention, the term "cylinder end
structure" is used to refer to a physical body at either end of the
linear path of piston reciprocation with which the piston, or
structures linked to and oscillating with the piston, can collide
if the piston's amplitude or oscillation increases excessively. The
term "piston drive" or "drive" is the driving force or power
applied to the piston to force it in its reciprocating, linear
oscillation. Since piston amplitude is an increasing function of
piston drive, an increase or decrease in piston drive, respectively
increases or decreases the amplitude of piston oscillation if other
parameters remain constant or undergo only variations which do not
completely negate the change in piston drive.
Therefore, in summary, workers in the free piston Stirling machine
industry have recognized the need for including, either within the
control system or as auxiliary structures, a means for preventing
free piston machines from damaging or destroying themselves by
internal collisions. Prior art free piston machine control systems
have focused on the steady state operation of the machine. During
this steady state operation, if an initial collision is not
recognized and the control system does not make the proper
corrections, the magnitude of the collisions can increase with each
cycle, often leading to irreparable damage. Such conventional
control systems have not attempted to maximize piston stroke, and
thereby minimize the time until the machine reaches steady state
operating conditions, during the transient cool-down period and at
the transient temperature conditions which occur after a free
piston machine begins operation and while it works toward steady
state operation. Control systems adapted to handle the unique
conditions experienced in the transient temperature range are
particularly important for free piston coolers.
It is, therefore, an object and feature of the present invention to
provide a control system for a free piston cooler, which, under all
operating conditions including transient start up conditions,
maximizes piston stroke and therefore minimizes the transient time
period, while avoiding collision of the piston, or component
structures reciprocating with the piston, against cylinder end
structures.
BRIEF SUMMARY OF THE INVENTION
In one of the many preferable configurations, the transient
temperature control system and method of the present invention
controls a free piston cooler, having a cold head and a warm end
and includes a cold head temperature sensor, a relational
interface, and a temperature controller. The cold head temperature
sensor senses the temperature of the cold head and generates a
temperature signal. The relational interface is in communication
with the temperature signal and contains a predetermined
relationship between the cold head temperature and a maximum piston
stroke during the transient cool-down temperature range. The
relational interface generates a transient range maximum allowable
stroke signal from the temperature signal and the predetermined
relationship. The temperature controller preferably also controls
the stroke of the piston while the cold head operates at a steady
state cold head temperature. The temperature controller is in
communication with the relational interface and is capable of
receiving the transient range maximum allowable stroke signal and
using it in limiting the piston stroke to prevent collisions within
the cooler during the transient cool-down temperature range.
The relational interface contains the predetermined relationship
between the cold head temperature and the maximum piston stroke
during the transient cool-down temperature range. The predetermined
relationship may be determined via computer modeling of the free
piston cooler or through experimentation. The relationship may be
established by operating the free piston cooler during the
transient cool-down temperature range and recording the stroke that
results in a collision, referred to as a collision stroke, between
cooler components. A stroke reduction factor is then applied to the
collision stroke, to provide a safety margin, to produce a
transient controlled stroke used in generating the transient range
maximum allowable stroke signal. The predetermined relationship is
often effectively stored as a plurality of data, which may be
expressed in tabular form, or as an algorithm.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Without limiting the scope of the present invention as claimed
below and referring now to the drawings and figures:
FIG. 1 is a partial cross section view, not to scale, of an
embodiment of a free piston cooler that is well known in the prior
art; and
FIG. 2 is a schematic of the transient temperature control system
of the present invention.
FIG. 3 is a table showing an example of the data values stored in
the relational interface of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The detailed description set forth below in connection with the
drawings is intended merely as a description of the presently
preferred embodiments of the invention, and is not intended to
represent the only form in which the present invention may be
constructed or utilized. The description sets forth the designs,
functions, means, and methods of implementing the invention in
connection with the illustrated embodiments. It is to be
understood, however, that the same or equivalent functions and
features may be accomplished by different embodiments that are also
intended to be encompassed within the spirit and scope of the
invention.
With reference generally now to FIG. 1 and FIG. 2, in one of the
many preferable configurations, the temperature control system 50
incorporates a free piston cooler 100, having a cold head 110 and a
warm end 120, a cold head temperature sensor 200, a relational
interface 300, and a temperature controller 400. The temperature
sensor 200 may be in thermal connection directly to the cold head
110 or connected to or within the insulated enclosure which
conventionally surrounds the cold head and contains one or more
objects to be cooled.
The free piston cooler 100 of FIG. 1 includes a piston 130, driven
by a piston driver 132, reciprocating within a cylinder 150, and a
displacer 140 attached to a displacer rod 142, slidingly passing
through the piston 130 and attached to a displacer spring 144. The
displacer 140 reciprocates within the cylinder 152, indicated by
motion indicator Md, between the cold head 110 and the warm end
120. The preferred piston driver 132 is a linear motor having an
armature winding 133 which drives magnets 135 which are fixed to
the piston 130. A regenerator is contained within the displacer.
This arrangement is described in more detail in U.S. Pat. No.
6,446,336 which is herein incorporated by reference.
During operation of the free piston cooler 100, the piston driver
132, typically an electric linear motor, moves the piston 130 in
the directions of motion indicated by Mp. The movement of the
piston 130 from a first position to a second position, where the
direction of travel reverses, defines a stroke, also referred to as
amplitude. The piston stroke of the free piston cooler 100 is
variable, is a function of the amplitude of the electromagnetic
field generated by armature winding 133 and is variably controlled
by the control system to satisfy any number of objectives.
Efficient operation is accomplished in the steady state by
operating the piston at a stroke which maintains the temperature of
the objects being cooled at a set point temperature.
In operation, a working fluid, generally helium, is transported,
compressed, and expanded by the combined movement of the piston 130
and the displacer 140. As previously mentioned, the movement of the
piston 130 is effected by the piston driver 132. The motion of the
displacer 140 is the result of many combined actions including, but
not limited to, the pressure wave resulting from changes in the
working fluid pressure created by the movement of the piston 130,
damping effects in the free piston cooler 100 introduced by the
working gas density and friction, the displacer spring 144, and
other components. The movement of the displacer 140, indicated by
Md, shuttles the working fluid between the cold head 110 and the
warm end 120, generally through a working fluid passage 160 and a
regenerator contained within the displacer. The regenerator
consists of an energy storage medium to and from which the working
fluid may transfer energy as it cycles from the cold head 110 to
the warm end 120, and back again. Modem regenerators may
incorporate pieces of fine porous metal and prevent unnecessary
heat loss and improve efficiency. Heat, indicated by Q, is absorbed
at the cold head 110 during expansion of the working fluid and
heat, Q, is rejected at the warm end 120 during compression of the
working fluid. Heat exchangers are generally attached to the cold
bead 110 and the warm end 120 to improve the transfer of thermal
energy to, and away from, the free piston cooler 100.
The free motion of the piston 130 in the free piston cooler 100 is
both a beneficial attribute and a source of potential problems.
Free piston machines are subject to collisions between the piston
130 and the displacer 140 and of either with other internal
components of the free piston cooler 100, such as cylinder end
structures, often resulting in damage. As the technology has
advanced, numerous control systems have been introduced to minimize
the destructive collisions.
The transient temperature control system 50 and method of the
present invention accounts for the unique characteristics of the
transient cool-down temperature range, thereby eliminating
destructive collisions and permitting the free piston cooler 100 to
operate at the maximum safe stroke to achieve rapid cool-down. The
embodiment of the invention illustrated in FIG. 2, includes a
conventional feedback control loop for controlling the piston drive
and therefore the piston amplitude during steady state operation in
a conventional manner. The control system 50 includes the cold head
temperature sensor 200 to provide a temperature feedback signal,
and the temperature controller 400. The cold head temperature
sensor 200 senses the temperature of the cold head 110 and
generates a temperature signal 210. This temperature signal 210 is
applied to the temperature controller 400. The temperature
controller 400 has cooler control logic 212 to which the
temperature signal 210 is applied for use in the conventional
feedback control loop to control the temperature of the cooler 100
during steady state operation. The stroke command signal output
from the cooler control logic 212 is applied to a stroke limiter
214 which, during steady state operation, limits the stroke command
signal to confine it to a stroke which is appropriate for steady
state operation. The stroke command output from the stroke limiter
212 is then applied to conventional stroke control logic 216 which
control the drive power and force applied to the piston drive
132.
Additionally, for purposes of the invention in controlling the
piston stroke during the transient cool-down period, the control
system also includes a relational interface 300 in communication
with the temperature signal 210 to receive the temperature data.
The relational interface 300 contains a stored, predetermined
relationship between the cold head temperature and a maximum piston
stroke during the transient cool-down temperature range. The
relational interface 300 generates a transient range maximum
allowable stroke signal 310 from the temperature signal 210 and the
predetermined relationship. The temperature controller 400 is in
communication with the relational interface 300 and receives the
transient range maximum allowable stroke signal 310 and limits the
piston stroke to the stroke it receives from the interface 300 to
prevent collisions within the cooler during the transient cool-down
temperature range.
The cold head temperature sensor 200 may be of virtually any
temperature sensing technology that can accurately sense
temperature in the transient temperature cool-down range. In fact,
the cold head temperature sensor 200 is preferably a sensor used by
the temperature controller 400 for steady state operation, and does
not have to be a separate and unique sensor. Sensors suitable for
such operation include diode type sensors and resistance
temperature detector type sensors.
The relational interface 300 contains the predetermined
relationship between the cold head temperature and the maximum
piston stroke during the transient cool-down temperature range.
Although the predetermined relationship might be determined via
computer modeling of the free piston cooler 100, it is determined
more effectively through experimentation. In one embodiment, the
predetermined relationship is stored electronically in a memory
device. Additionally, the predetermined relationship is often
effectively stored as a plurality of data. The plurality of data
may be expressed in tabular form, possibly in a database.
Alternatively the data may be expressed as an algorithm, such as a
series approximating a plot of the tabular data.
In the embodiment wherein the predetermined relationship is
experimentally determined, the relationship may be established by
operating the free piston cooler 100 during the transient cool-down
temperature range, manually controlling the piston stroke and
recording the stroke that results in a collision, referred to as a
collision stroke, between cooler components. This process may be
repeated for any number of temperatures within the transient
cool-down temperature range. For example, a transient cool-down
temperature range that begins with the first operating temperature
of approximately 300.degree. K and the set point operating
temperature of 70.degree. K may have experimental data collected at
10.degree. K intervals over the entire range resulting in 24 pairs
of temperature and collision stroke values. A stroke reduction
factor is then applied to the collision stroke, to provide a safety
margin, to produce a transient controlled stroke prior to
generating the transient range maximum allowable stroke signal 310.
As one with skill in the art will recognize, such experimentally
collected data may also be configured as an algorithm, such as by
using a mathematical series to approximate a curve generated by the
collected data.
The temperature controller 400 receives the transient range maximum
allowable stroke signal 310 for use in practicing the invention.
The temperature controller 400 may be virtually any of a number of
known control systems designed to control the operation of a free
piston cooler 100 and prevent destructive collisions therein. One
such temperature controller 400 is shown schematically in FIG. 2.
In this particular embodiment, the temperature controller 400
consists of the cooler control logic device, stroke limiter, and
stroke control logic devices connected to form a feedback control
system as described above. The cooler control logic device 212
typically receives a set point operating temperature Ts and a
measured operating temperature, and generates a stroke command. The
stroke command is applied to the stroke limiter 214. The stroke
limiter may reduce the value of the stroke based upon any number of
occurrences typically associated with a collision between
components of the free piston cooler. One method for detecting
collisions within free piston coolers has been to acoustically
sense collisions and reduce the stroke command by a predetermined
value. Alternatively, stroke limiters have also relied upon
velocity and acceleration detectors to detect collisions by
detecting a high rate of piston deceleration, which exceeds piston
deceleration during normal machine operation. Further, stroke
limiters have also simply incorporated the use of a limit switch to
detect destructive collisions. Next, the stroke control logic
device receives the stroke command from the stroke limiter and
associates it with a level of power to transfer to the cooler,
typically to the piston driver 132, or electric linear motor.
The temperature controller 400 prevents collisions during the
transient cool-down temperature range by receiving a separate
input, reflective of the transient conditions, that may in essence
override the temperature controller's steady state stroke command.
This override signal is the transient range maximum allowable
stroke signal 310 generated by the relational interface 300. The
temperature controller 400 then limits the stroke command so as not
to exceed the transient controlled stroke value, from the
predetermined relationship, for any given cold head temperature
during the transient cool-down temperature range. As such, the
temperature controller 400 is capable of preventing collisions
during the transient cool-down temperature range as well as the
later period of steady state temperature operation.
FIG. 3 is a table illustrating a representative example of the data
which is stored in the relational database of the present invention
for an embodiment of the invention. The maximum piston stroke
values represent the piston stroke limits which are input to the
stroke limiter 214. Each of the data input to the stroke limiter is
a value which represents the illustrated stroke limit which is
input for the illustrated temperature shown in the table. Because
the temperature controller 400 is designed to control the cooler at
normal operating temperature, the stroke dimension stated in the
table represents data for limiting the piston to the stated stroke
dimensions at steady state operating temperature. Consequently,
data from the relational database which limits-the stroke to a
particular dimension at steady state temperature, will allow a
greater stroke at a higher, cool-down period temperature because
there is less damping. Therefore, the actual piston stroke at the
temperatures shown in the table may all be identical, but the
stroke limitation signal is changed to greater stroke limitations
as the cooler cools down and damping increases. In other words, as
the cooler cools down and damping increases, the stroke limitation
is increased but the actual stroke itself can be maintained at a
constant maximum as cool down occurs.
Numerous alterations, modifications, and variations of the
preferred embodiments disclosed herein will be apparent to those
skilled in the art and they are all anticipated and contemplated to
be within the spirit and scope of the instant invention. For
example, although specific embodiments have been described in
detail, those with skill in the art will understand that the
preceding embodiments and variations can be modified to incorporate
various types of substitute and or additional or alternative
materials, relative arrangement of elements, and dimensional
configurations. Accordingly, even though only few variations of the
present invention are described herein, it is to be understood that
the practice of such additional modifications and variations and
the equivalents thereof, are within the spirit and scope of the
invention as defined in the following claims.
* * * * *